1. Field of the Invention
The present invention relates to a lens device where a lens used for a camera or the like is inserted and fixed in a lens-barrel.
2. Description of Related Art
A digital camera, a mobile phone camera and so on use a lens device in which one or a plurality of lenses are inserted and fixed in a lens-barrel. Such a lens device is manufactured by performing core alignment so as to align the optical axes of a lens and a lens-barrel and the optical axes of a plurality of lenses and then inserting and fixing the lenses into the lens-barrel.
In such a lens device, a plastic lens, a plastic lens-barrel and so on are used because they are low cost and easy to form a desired shape.
The plastic lens and lens-barrel, however, are subject to expansion and shrinkage due to temperature or humidity change. Thus, even if core alignment is performed when inserting and fixing the lens into the lens-barrel to assemble them, the misalignment of the optical axes between a lens and a lens-barrel or between lenses can occur upon temperature or humidity change, which results in deterioration in optical properties.
In order to solve the above problem, Japanese Unexamined Patent Application Publication No. 04-204408 proposes a lens device with a plastic lens and a lens-barrel for holding the lens. The lens device has at least three protrusions that are formed on the inner surface of the lens-barrel to project toward the center axis (optical axis) of the lens-barrel so as to hold the lens with their ends in contact with the periphery of the lens. The diameter of the circle which the protrusion ends inscribe is smaller than the outer diameter of the lens that is supported by the protrusions. Further, the rigidity of the lens-barrel is smaller than the rigidity of the plastic lens. The lens device thereby prevents the decentering or distortion of the lens under change in dimension due to environments such as temperature and humidity.
According to this proposed technique, the center of the lens always receives the force that corresponds to the center of the lens-barrel, thereby preventing the decentering and holding the lens at a given position in the lens-barrel. It is, however, still unable to suppress the misalignment between the lens and the lens-barrel and between the lenses sufficiently.